Cell Biology and Genetics of T Cell Activation and Death At the initiation of viral or bacterial infection antigen-specific T cells are programmed to proliferate and accumulate to very large numbers. The magnitude of proliferation sums the rates of cell division and cell death, and it is dependent on the persistence of antigen, growth factors, nutrients, and stress. This T cell expansion plays an important role in the clearance of infectious agents, but with characteristic kinetics, the population undergoes an equally dramatic decline to a small steady-state population of memory T cells. Experiments are proposed to test the hypothesis that T cells have co-evolved with natural pathogens to avoid virulence mechanisms aimed at the inhibition of cell death. Select caspases (cysteine proteases) mediate several aspects of cellular suicide leading to apoptosis, and yet inhibition of Caspase-8, a form of viral evasion, results in an alternate program of cell death. We have now determined that Ripk1 is a sensor of caspase-8 activity, and mediates an alternate form of cell death in the absence of Caspase-8. Experiments are proposed to characterize the different forms of cell death occurring at various phases of the immune response with or without a Caspase-8 deletion. The approaches include genetic complementation with genes known to regulate apoptosis, necrosis and autophagy, and morphological analyses using transmission electron microscopy and confocal immunofluorescence. Understanding the biology underlying T cell expansion and cell death is important in designing new immunologically-based therapeutic strategies, especially for infectious agents mediating chronic diseases. PROJECT NARRATIVE: One of the most important mechanisms of medical intervention relies on vaccination against infectious disease, yet many diseases are resistant to our standard vaccine strategies. In particular, the evasion strategies of infectious agents often include the inhibition of cellular suicide mechanisms necessary for an effective and well-regulated immune response. This proposal explores the biology and genetics of cell death in the immune system with a particular emphasis in understanding how apoptosis, necrosis, and autophagy contribute to T cell death.